Identification by means of radio signals (RFID (=Radio Frequency Identification) has been used for many years for various applications. RFID systems can be designed for different frequency ranges. Low-frequency (LF) systems operate, for instance, in the 120 kHz band, while high-frequency (HF) systems can operate, for instance, in the 13.58 MHz band. Microwave systems (MW) operate in the GHz range.
A recent trend in the RFID branch is ultrahigh frequency (UHF) RFID in, for instance, the 860 to 950 MHz band. However, there are also UHF systems known that operate in other UHF bands, such as, for instance, the 400 to 450 MHz band.
The invention relates to UHF RFID devices.
RFID tags, also called labels, passes, detection plates, responders, badges, transponders, etc., exist in many divergent shapes and dimensions. Such tags can be of the active, semi-active or passive type.
Active tags have a battery and typically emit their identification signal intermittently. Semi-active tags also have a battery, but emit their identification signal only in response to a signal received from a transceiver device, also referred to as interrogator and referred to as reader in the following description and claims. Such a signal can be an interrogation field, also called detection field, that is broadcast continuously by the reader, but can also be an intermittently broadcast field. Passive tags have no battery and utilize the energy emitted by a reader to provide their RFID chip with supply energy. The invention is in particular, though not exclusively, suitable for use in access control systems, which utilize passive tags.
Low-frequency passive tags can be read out contactlessly over distances of up to about 1 meter. Passive high-frequency RFID tags generally have a shorter maximum reading distance of a few decimeters. Active or semi-active tags can be read out at a greater distance than passive tags. A readout distance of 10 m is no exception. A drawback of the use of active or semi-active tags is that these tags are more complicated than passive tags and hence considerably more expensive. Also, the necessity of using a battery in a (semi-)active tag is a drawback.
Users of access control systems, such as, for instance, person access control systems, have a need for systems that are suitable for passive tags, while yet allowing a handsfree use of the system. ‘Handsfree’ in this connection means that the tag does not need to be presented to the reader at a very short distance and/or in a predetermined orientation. In the concrete, for a person access control system, this means that a person carrying a pass belonging to the access control system can leave it attached to his clothes, or leave it hanging from a cord around his neck, or the like, and that the pass can then be recognized by the system nonetheless. However, this is always subject to the condition that the UHF pass has no direct contact with the person's body and is always in line of sight (LOS), whether or not blocked by clothes or the like, with the reader concerned.
The conditions for a proper operation of a system envisaged as a handsfree RFID reader system include a great reading distance and a free orientation of the tags (labels, passes, detection plates, responders, transponders, etc.). Free orientation in this connection means complete rotational freedom about the line of sight (LOS), and a limited rotational freedom about axes perpendicular to the line of sight up to an angle in the order of 45° with respect to the line of sight.
To enable tag detection independent of the orientation of the tag, the antenna system of the reader should have an antenna gain and a band width that are approximately equal for two orthogonal polarization directions. In a practical situation, the two polarization directions will typically be formed by a horizontal and a vertical field component.
In addition, there is a need for the smallest possible dimensions of the readers. In practice, users ask for a reader that fits on a standard door frame post. This is to say that such a reader in practice should have a width in the order of 5 cm or less.
While LF and HF readers are available in small dimensions, fitting on a door frame post, these known readers have a very limited reading distance of only a few decimeters.
Summarizing, therefore, a need exists for a reader for a UHF RFID access control system, where the reader is to be suitable for handsfree use of tags belonging to the UHF RFID system, where it must be possible for the tags to be of the passive type and to be read out at a relatively large distance in the order of a few meters, and where the reader has a small width, which, in the case of use in e.g. an access control system, for persons, fits on a door frame post of conventional dimensions.